12 years ago, a strange enemy from another dimension arrived and destroyed Tokyo who was almost completely immune to traditional weapons. To combat this alien menace, the government built the giant robot Dai-Guard. Now, years of peace have made Dai-Guard a useless anomaly, and the government foists the robot off on the 21st Security Corporation, a civilian corporation. However, as Dai-Guard's new owners cleaning out its cobwebs, a new alien menace arrives who overpowers the traditional military. Deciding he must protect his beloved city no matter what, Akagi Shinnosuke takes two of his employees and fires up the aging and malfunction prone Dai-Guard to save the city.